Known in the prior art are several different types of washing methods and apparatus therefor. Arrangements clearly differing from each other are drum washers, fourdrinier washers and diffusers. The feed consistency of drum washers and fourdrinier washers is normally between 1 and 3%. Drum washers in use at the moment are, for example, suction washers, washing presses and pressure washers. The pulp is fed into the diffuser washers at a consistency of about 10%.
A conventional suction washer comprises a wire coated cylinder which is rotatably mounted in a vat. The casing of the cylinder is provided, below the perforated plate, with a plurality of collecting compartments which are each connected through a separate pipe to a valve system located on the shaft at the end of the drum. The filtrate is guided from the valve through a suction leg to the filtrate chamber. Due to the valve construction it is possible to obtain the proper suction effect at different points of the web formation.
The web formation in the suction washer is carried out in such a way that by means of a suction duct an under-pressure is generated in the cylinder rotating in the vat, which under-pressure draws pulp from the vat against the cylinder. The consistency of the fiber suspension in the vat is about 0.5 to 2%, and the consistency of the layer thickened on the cylinder surface is about 10 to 12%. The web formation area, in other words, the part of the cylinder surface which is covered by the fiber suspension in the vat, is about 140.degree.. The maximum rotational speed of the cylinder is 2 to 2.5 r/min., since at higher rotational speed the collecting compartments and pipes for the filtrate do not have sufficient time to empty.
The washing is carried out as a displacement washing in such a way that the washing liquid is sprayed onto the surface of the cylinder which is visible outside the pulp vat. The washing liquid due to the under-pressure is drawn through the pulp layer thereby displacing most of the chemicals containing liquid therein. Consequently, the extent of the displacement area is about 120.degree.. The typical specific square load of a vacuum washer in bleaching use is 5 BDMT/m.sup.2 /d, whereby the thickness of the pulp web is about 25 mm. In the bleaching use the square load of a vacuum washer is about 8 BDMT/m.sup.2 /d and the thickness of the web is about 30 mm.
The washer press comprises a wire coated or drilled cylinder with a perforated plate casing. The pulp is fed at a consistency of 3 to 4% and any foreign matter and impurities such as knots and the like must be removed from the pulp prior to the washer. The casing of the cylinder includes compartments from which the filtrate is guided out through a chamber in the end plate of the cylinder. The cylinder may also be open in such a way that the filtrate accumulates inside the cylinder and is discharged through an opening at the end of the cylinder. The length of the web formation zone is about 90.degree. and that of the displacement zone 150.degree.. The rotational speed is about 2 r/min. and the specific square load is 15 to 20 BDMT/m.sup.2 /d. The consistency of the washed pulp may rise even to 30% when a press roll is used. The displacement, however, is carried out when the pulp web is about 50 mm thick and has a consistency of 10%.
An apparatus in accordance with U.S. Ser. No. 921,786 discloses a pressure washer which comprises mainly a drilled and perforated plate cylinder on the surface of which 50 to 60 mm high stripes are mounted at intervals of about 200 mm. Filtrate compartments are disposed beneath the pulp compartments within the casing of the cylinder. A valve system is mounted to the outer rim at the end of the cylinder, through which system the filtrate is guided out. The washer may have 3 to 5 stages, in other words the filtrate is guided upstream by pumping from one stage to another. The washing liquid spaces between the respective stages are sealed.
The web formation is carried out in such a way that the pulp being washed is first fed into a feed box. The bottom of the feed box is formed by a perforated plate above which an endless wire cloth is mounted. The feed box is tapered towards the washing cylinder. When the pulp is in the feed box liquid is discharged therefrom through the wire cloth and the perforated plate, whereby the pulp is thickened on the wire cloth. While the wire cloth is moving towards the cylinder, liquid is continuously discharged from the suspension and also due to the pressure caused by the tapered construction of the feed box. At the end of the feed box the pulp is guided into the compartments between the stripes of the washing cylinder, thereby forming axial "pulp planks" or cakes having the length of the cylinder. Immediately subsequent to the feed point the cylinder is provided with the first washing zone. The washer disclosed in said publication has five separate zones. A washing liquid flow is guided to each zone and pressed through the pulp layer in the compartments of the washing cylinder thus displacing the chemicals containing liquid previously present therein. As mentioned above, the filtrates are guided upstream from one zone to another. In other words, clean washing liquid is pumped into the last washing zone and the displaced filtrate is guided into the second last zone to operate as washing liquid. Subsequent to the last washing zone, the "pulp planks" are removed from the cylinder, for example, by blowing compressed-air, and are transferred further by a conveyor screw.
The specific square load of this type of pressure washer with four stages is about 2.4 BDMT/m.sup.2 /d. The thickness of a "pulp plank" is about 55 mm, and the consistency may rise even up to 15-17%. The washing water running from the compartment, however, decreases the consistency to 10-12%. The consistency of the pulp being fed to the washing cylinder is 3 to 6%. The rotational speed of the cylinder being used is about 0.3 rpm.
Typically in all apparatus described above, the consistency of the pulp being fed to the washer is relatively low, i.e. 6% at the maximum. Consequently, the pulp has to be diluted prior to the washing process from the consistency of 10 to 15% of the preceding treatment zones to less than half. Thus the amount of liquid included in the pulp at least doubles. If it were possible to carry out the washing at a high consistency, savings will be gained both, in the size of the apparatus and in the consumption of energy, as well as in the amount of filtrate guided to the evaporation. The problem is, however, that until now there does not exist an apparatus, by which pulp having a high consistency, i.e. over 6%, can be fed to the washer. On the other hand, it is also known that the higher the consistency of the pulp becomes, the higher will be the air content of the suspension which, in turn, will cause foam problems in the washing process. Corresponding problems are encountered also with other pulp treatment apparatus, for example, with thickeners.
Some of these problems have been partially solved, for example, in an arrangement disclosed in U.S. Pat. No. 4,468,319, in which the washing of the pulp may be carried out at the consistency of 6 to 14%. The apparatus is called a stationary diffuser because it is designed to replace the continuously operating diffusers based on the use of movable filter surfaces (e.g. U.S. Pat. No. 3,372,087). The arrangement in accordance with U.S. Pat. No. 4,468,319 is characterized in that it comprises stationary annular filter surfaces mounted inside the cylindrical outer casing and respectively annular feed means of washing liquid arranged between them. The annular spaces generated in the above described way are divided by radial plates into sector-like parts which form the flow passages of the pulp. The pulp being treated is brought to the bottom part of the substantially conical apparatus through the rotating joint part at the top of the apparatus which part is mounted to a nozzle rotating with the shaft of the apparatus. The sectional surface area of the nozzle corresponds to the combined sectional surface area of the flow ducts formed by each separate sector. Thus, when rotating the sector-like nozzle feeds pulp into each sector formed by several axial flow ducts. Otherwise, the lower ends of the sectors have been sealed by a plate seal rotating with the nozzle.
The washing liquid which is introduced into the apparatus through a hollow, rotating shaft, is guided from the feed apparatus on the opposite surfaces relative to the filter surfaces of the pulp rings, whereby the washing is carried out as displacement washing, i.e. the washing liquid pushes the liquid including dissolved chemicals towards the filter surfaces and therethrough towards the discharge ducts.
The discharge of the treated pulp is also carried out by rotating members which are mounted to the screw feeder.
Although the above described stationary diffuser has enabled the washing of pulp at a higher consistency, the above described apparatus has a number of disadvantages that have prevented a greater success of the apparatus. First, the construction of the apparatus is relatively complicated due to the presence of several connections for the pressurized washing liquid and the pulp is fed and washed in a pressurized state. Additionally, the connections are rotatable and thus difficult to seal. Second, when pulp is discharged from the apparatus, a screw feeder is still required to transfer the pulp further to the discharge vat.